Retrieve a field from an existing entity

One common use case I find is to need to get a field from an existing entity. Sometimes that field is from the destination entity, but other times it is just a field on a related entity on the destination site.

It been a while since I have been using code similar to the one I will present on a related merge request.

migrate_plus sounds like the right place for that process plugin to live in.

There is a bit of overlap with entity_lookup, but I still would suggest to have a different plugin, since the new one has a lot less magic, and is intended more as an "utility" plugin.

cc: @benjifisher